Oil and gas producing wells and oil refineries produce waste water streams that contain about 500 ppm or more residual water insoluble oil. In the oil industry, flotation is used to remove oil from waste water streams. Flotation involves contacting the oil droplets with gas bubbles. Due to the hydrophobic nature of oil, oil will attach itself to the gas bubbles which are rising through the oily water. The gas bubbles with oil attached rise to the top of the water. The gas is released, leaving the oil on top of the water, from where the oil is removed.
Conventional flotation equipment for the separation of oil from water comprise horizontal tanks provided with mechanical equipment to induce gas. This equipment produces an oily product containing about 3 to 5% oil with the balance being water. This product requires further treatment, e.g. by means of a gravity separator or a cyclone, to obtain a purer oil product. In addition, the treated water produced by the conventional mechanical flotation processes still contains trace amounts of oil which in some instances do not meet the stringent requirements prescribed by modern environmental legislation.
Although oil removal is the main problem, solids present in the water can be a serious problem as well. For example, if hydrogen sulphide gas (H.sub.2 S) is present in the water, FeS forms which is a fine black precipitate that is difficult to remove. The amount of precipitate varies with conditions but in some cases large quantities can form so that the water will be black even if all the oil has been removed. In addition, other precipitates can also form, such as calcium sulphate (CaSO.sub.4), calcium or magnesium carbonate (CaCO.sub.3, MgCO.sub.3) and barium sulphate (BASO.sub.4). If these solids are present, they can cause disposal problems and can detrimentally affect the appearance of the water. The solids are normally very fine, typically less than 10 microns, because they are precipitates, and are difficult to remove.
It is accordingly an object of the present invention to provide a flotation process for the treatment of oil contaminated water which produces a purer treated water product which can be discharged directly to the environment, if required, and an oil product which has significantly less water than the oily product produced by the conventional flotation methods.